1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a semiconductor device in which semiconductor elements are formed on a semiconductor substrate having element isolating regions provided by a field oxide film.
2. Description of the Background Art
LOCOS (Local Oxidation of Silicon) method has been used as a method of forming a field oxide film. Japanese Patent Laying-Open No. 4-234146, for example, discloses the conventional LOCOS method applied to a process for forming an element isolating insulating film in a DRAM (Dynamic Random Access Memory). One example of the method of forming a field oxide film by the conventional typical LOCOS method described in this laid-open application, for example, will be described with reference to FIGS. 14 to 18.
In the method of forming the field oxide film by the conventional LOCOS method, first, referring to FIG. 14, a silicon oxide film 102 having a thickness T.sub.1 of 100.ANG. to 300.ANG. is formed on a silicon substrate 101. Thereafter, a polycrystalline silicon film (or an amorphous silicon film) 103 having the thickness of 300.ANG. to 700.ANG. is formed on silicon oxide film 102. Further, a silicon nitride film 104 is formed on polycrystalline silicon film 103. A resist 105 is patterned, and anisotropic etching is performed as shown in FIG. 15, so that silicon oxide film 102 is exposed. Thereafter, resist 105 is removed. Referring to FIG. 16, the exposed oxide film is further oxidized by thermal oxidation, for example, to form a field oxide film locally having a large thickness T.sub.2 of 5000.ANG. to 6000.ANG.. Thereafter, referring to FIG. 17, silicon nitride film 104 and polycrystalline film 103 are removed. When a flash memory, for example, is to be formed on silicon substrate 101 including field oxide film 110 formed in this manner, a conductive layer 108 which will be a floating gate electrode is formed with an end portion 108 positioned on field oxide film 110, as shown in FIG. 18. Thereafter, referring to FIG. 19, an ONO (Oxide-Nitride-Oxide) film 114 providing inter-electrode multi-layered insulation film is formed to cover the surface of conductive layer 108 and, a conductive layer 115 which is to be the control gate electrode is formed further thereon. Thereafter, an interlayer insulating film 116 and a bit line 117 are formed further thereon, whereby a flash memory such as shown in FIGS. 20A, 20B and 20C is completed.
However, the method of forming an element isolating insulating film by the LOCOS method suffers from the following problem.
In the method of forming field oxide film 110 by the LOCOS method, it is necessary to increase thickness of silicon nitride film 104 in order to suppress a bird's beak 110b formed below silicon nitride film 104 near an edge of an opening 120, as shown in FIG. 16. This results in a large concave at a region 111 between oxide film 102a(thickness: T.sub.1) deposited thin and a bird's head 110a of field oxide film 110 which is made very thick to maximum thickness of T.sub.2 by thermal oxidation.
Accordingly, when conductive layer 108 for the electrode is formed having end portions 108 positioned on silicon oxide film 102a and field oxide film 110 as shown in FIG. 18, end portion 108a of conductive layer 108 for the electrode extends upward along a steep inclination near the region 111, and hence end portion 108a of conductive layer 108 protrudes with a sharp angle. This may possibly cause electric field concentration at end portion 108a of conductive layer 108, resulting in dielectric breakdown and emission of charges in conductive layer 108 from end portion 108a. As a result, application of such conductive layer 108 to the floating gate electrode of a flash memory shown in FIGS. 20A to 20C, for example may possibly lead to a problem of degraded charge retention characteristic.